DE3544005A1 - DEVICE FOR CONTROLLING THE DISTANCE OF A MELTING ELECTRODE TO THE SURFACE OF THE MELTING MATERIAL IN A VACUUM ARC FURNACE - Google Patents

Description

The invention relates to a device for regulating the distance Melt electrode to the surface of the melt in a vacuum light Bow furnace according to the preamble of claim 1.

For the production of high quality metals and metal alloys that have as little foreign body inclusions and homo structure different methods are known. One of the best known The process is arc melting, in which an electrode is closed extends a crucible and by applying an electrical voltage the electrode melts at its tip between the electrode and the crucible and falls into the crucible as a liquid material. As a rule, the so-called called melting electrode to the one pole of a DC voltage and the Crucible placed on the other pole of this DC voltage. However, there are superimpositions with alternating voltages are also possible in order to to achieve effects.

A major problem in the operation of the arc melting furnace of the above is to regulate the arc length, i.e. the Ab stood between the bottom of the electrode and the surface of the melting material already in the crucible. The arc is closed long, the electrode and / or the melting material can be heated incorrectly be so that the quality of the melting material is greatly reduced. There on the one hand, the level of the melting stock in the crucible is continually increasing and on the other hand the distance between the end of the electrode and do not immediately observe the surface of the crucible during operation can be respected, special measures must be taken to to regulate this distance.

In arc melting furnaces that operate at atmospheric pressure or only slightly work below it, the arc length is regulated by the fact that a predetermined arc voltage is maintained. At atmosphere The plasma arc is characterized by a pressure voltage gradients, e.g. 20 volts per 2.5 cm. The Voltage drops on the cathode and on the anode surface are increasing  together an additional 20 volts, so that when you have a light arc length of e.g. Wants to maintain the electrode in one brings such position that the arc voltage is 30 V. This can can be easily realized by means of conventional devices which Measure and regulate arc tension.

However, the method described above cannot always be used for arc melting furnaces which operate in a vacuum. Such arc melting furnaces are used in particular for melting the so-called refractory active metals such as titanium or zircon, and for the production of stainless steels and high-temperature alloys. When the gas pressure surrounding the arc decreases, the voltage gradient of the arc plasma also decreases, and at very low pressures the voltage gradient of the arc plasma can be, for example, only one volt per 2.5 cm. Since the anode and cathode voltage drops, for example in steel, are approximately 20 volts, the voltage at the arc is very small compared to the other voltage drops. Changes in gas content and alloy composition affect the anode and cathode voltage drops on the order of the voltage drop in the "arc column". As a result, the method of regulating the arc by keeping the arc voltage constant, especially in the case of steels, is associated with major errors; ie the actual length of the arc will usually deviate greatly from the desired length.

In the meantime, an arc melting furnace is already known which is used for the rain the distance between the electrode and the surface of the melting material from the Knowledge makes use of that tension even when properly Operation breaks down at short intervals (US-PS 29 42 045). This effect is caused by short circuits caused by Liquid metal drops arise from the electrode in the crucible and briefly drop the electrode with the melting material in the crucible connect trically conductive. As long as the duration and frequency of this short conclusions are not very large, the arc works with almost full  Performance, so that no significant influence on the melting material heating takes place. If the arc becomes shorter, the frequency increases the bow shorts too.

According to the known arc melting furnace, the electrode gap regulated in such a way that the frequency of arc shorts within of a certain area. Here, for example, a Voltmeter is observed and the time intervals are shown with a stopwatch measured between the individual voltage dips so that the span dips in time per unit of time can be determined.

Another known device for regulating the electrode spacing in an arc melting furnace is based on the knowledge that light arc voltage voltage fluctuations in the form of positive rising Impulses are superimposed, each of which occurs during a short span of time ne of, for example, 40 milliseconds with a frequency of 30 Hz occurs (DE-PS 12 12 651). These voltage pulses, about their cause nothing is said are used to re-measure the electrode spacing apply, the voltage curve into a basic component and a two component is split. The in the second component as Voltage, current or impedance fluctuations occurring impulsively fluctuations are detected, and the electrode is dependent on the frequency of these fluctuations regulated. So it becomes a pulse count of the overvoltages per time unit and if the number of pulses is too low, the electrode is removed lowers.

In another known device for arc melting, oscilloscopic or graphical observations, the zei gene that briefly during the melting of metals in a vacuum Short circuits from 0.1 to 0.3 seconds between the electrode and the molten metal surface of the crucible occur. In addition, be takes into account that changes in the arc voltage occur which arise from impurities, which in turn indicate changes in  the composition or pressure of an inactive gas atmosphere rest or by striking an arc from the electrode are due to the crucible wall, these last mentioned tension changes are smaller than the voltage changes that occur when Drop of molten metal with the molten electrode Connect metal band (US-PS 29 15 572). This known device has a device with which the electrode points in the direction the surface of the metal in the crucible by an amount is at least equal to the difference between the melting rate of the electrode and the rate of rise of the metal surface. The Vorrich device also has facilities that are due to molten trop fen between the electrode and the metal surface in a predetermined activated position of the electrode in relation to the metal surface to determine the electrode away from the metal surface distance. The voltage short circuits are here detected by a relay that controls a timepiece.

Finally, another device for regulating the electrodes distance known, at which the drop short circuits between the elec trode and the liquid metal surface of the crucible as a control criterion (U.S. patent application by Sandia Corp., inventor: Fisher, Maroone, Tipping and Zanner). The drop shorts and the assigned voltage reductions appear as repetitive lending pulses that correlate closely with the electrode spacing. The number the drop short circuits are added up and each time the Number of short circuits has reached a predetermined value, the mean period between the shorts, starting from this predetermined value, as well as the time in which this value is enough, calculated. For this calculation and for the display of the A microprocessor is used for the duration of each short circuit. It will immediately after the pulse formation of the natural drop Short circuits worked digitally with a computer. The standardized impulse These are fed to an event register, the one to be counted Pulse amount previously entered via a computer control panel and fall  can be changed wisely. The content of the event register falls below sters the specified number of pulses, which is determined by coincidence, so a command is given to a time measurement register, and the elapsed time between the respective coincidences is read out. This value serves as a measure of the distance between the electrode and the liquid metal surface area. The measured value is in each case after reaching the predetermined An number of drop short circuits (approx. 100 short circuits) renewed. Such The device has two disadvantages, namely that the measured value is first is refreshed after relatively long periods of time, i.e. not up to date and that when little or no drops appear, time until a rule intervention becomes very long. If the number of drops is high, the Control intervention, however, very quickly. The determination times for the drops number represent a dead time. This dead time is for different loading drive states of different lengths. The time behavior of the measuring element is non-linear. The phase shift of the signal is thus instantaneous dependent on operating conditions. The size of the rule intervention at a Deviation from the setpoint must be severely restricted in order to get a swing to avoid. So a small loop gain must be chosen the. This requires a sluggish disturbance control with large deviation of the setpoint.

The invention is therefore based on the object of a device to create the preamble of claim 1 with which it is possible is to achieve improved electrode control.

This object is achieved in accordance with the features of patent claim 1.

The advantage achieved with the invention is in particular that the electrode controller can not only be set very sharply, son that it is also possible to have a very fast regulation with only a little to achieve deviations from the target value.

In this way, the invention optimizes the closed control loop, which has the disadvantage compared to an open control loop that only  a control deviation must occur until the controller has reached a manipulated variable can make a correction at all. In general control tech This disadvantage is usually eliminated by a so-called regulation Tried to eliminate interference surcharge. This is a disturbance variable measured and fed to the actuator via an auxiliary controller. When open If a disturbance variable occurs, a correction signal is generated immediately without wait until there is a rule deviation. This branch represents from which Disturbance seen here, an open chain, i.e. a controller with all their disadvantages.

An embodiment of the invention is shown in the drawing and is described in more detail below. Show it:

FIG. 1 shows the voltage waveform or the current waveform for a typical drop short-circuit;

Figure 2 shows a known control circuit for feeding an electrode in a crucible.

Fig. 3 shows a circuit arrangement according to the invention for the drop short circuit control in a melting electrode device in analog technology;

FIG. 4 shows a circuit arrangement according to the invention corresponding to FIG. 4, but in digital technology;

Figure 5a is a graph showing the functional relationship between gap width and dot string.

Fig. 5b is a graph showing the functional Abhängikeit between gap width and frequency drops.

In Fig. 1 is shown, in which way the current and voltage change by the flow between a anste melting electrode and a crucible hen or this route. It can be seen here that with a direct current which is superimposed on an alternating current with a small amplitude, the voltage U generally remains constant and only has a minimum in points A, B and a maximum in points C, D. If the voltage U has a minimum, then the current has a maximum, see points E and F.

The voltage dips in points A and B each indicate a short circuit, which is caused by a drop of liquid metal, which briefly connects the electrode to the surface of the molten metal in the crucible.

In FIG. 2 shows a known circuit arrangement is shown which is suitable for the drop short-circuit control. Using this known circuit arrangement, the differences and advantages of the inven tion can be better understood. The actual drop short-circuit control (= drop-short control) is designated by the reference number 20 . It contains several components and works as follows: After the galvanic separation of the arc voltage U _L by an isolating converter 21 , the voltage drops due to the drop short circuits are filtered out via a differentiator 22 . The limit frequency of the differentiating member 22 is designed so that the short circuits can be detected correctly. In the subsequent trigger circuit 23 , the filtered pulses are formed and converted in a subsequent monostable multivibrator 24 into standard pulses, ie into pulses of constant amplitude and width.

An integrator, more precisely a PT ₁ element 25 with a fixed integration time, forms the mean pulse value from the resulting normalized pulses. The output variable of this integrator 25 represents the short-circuit frequency actual value U _si , which is compared with a short-circuit frequency target value U _ss tapped at a potentiometer 27 and is supplied as a difference to the short-circuit frequency controller 26 . The corresponding digital ana logon for this would be the counting of pulses into a digital counter with a fixed time base and the evaluation of the pulse counter reading occurring within a fixed time interval. The number of drop short circuits occurring within a specified time is therefore counted. A ring counter could thus be used as the mean value generator, which determines the mean value over all the short-circuits detected in the ring time. The other circuit parts of FIG. 2 have nothing to do with the actual short circuit frequency control, although they are required for the overall regulation. You take into account other influencing variables on the control, because the drop short-circuits are only one of several possible control criteria, which is expressed by the summation element 28 . In addition to a constant voltage U _const , for example, the output variable of the short-circuit frequency controller 26 can be switched on via a switch 29 . Further, the summation element 28 may be applied to additionally via a switch 30 to the output of a general arc voltage regulator 31st This arc voltage regulator 31 is the difference between the arc voltage actual value U _Li and an arc voltage desired value U _LS, which is tapped from a potentiometer 32 via a link member 33 to. The output variable of this summation element 28 is connected to motor controllers via a controllable switch 34 . The switch 34 can be controlled via a relay 35 which is triggered by a voltage generated by a gas detector 36 . The motor controllers mentioned control two motors M 1 and M 2 , which are provided for the differential gear of the electrode. These motor controllers are of the same type and each have a P controller 37 or 37 ', an I controller 38 or 38 ', a pulse device 39 , 39 ', a rectifier 40 , 40 ' and a resistor, the Output of the resistor 41 , 41 'is fed back to a logic element 42 , 42 ' which lies between the P-Reg ler 37 , 37 'and the I-controller 38 , 38 '. Furthermore, the output variable of a tachodynamic TD ₁ or TD _{2 is} fed back to a logic element 43 or 43 ', the logic element 43 being connected between the tap of a potentiometer 44 and the P controller 37 , while the logic element 43 ' between the output of a Reversing amplifier kers 45 , which is also at the tap of the potentiometer 44 , and the P-controller 37 'is connected. Another motor M 3 of the differential gear of the electrode can be switched on via switches 46 , 47 , 48 , the switch 48 being controlled by a relay 49 , which in turn is controlled by a short-circuit resolution circuit 50 .

FIG. 3 shows a circuit arrangement according to the invention which corresponds in some details to the arrangement according to FIG. 2. The actual drop regulation is now done in a different way. To illustrate, in addition to the circuit arrangement mentioned, the crucible 60 is also shown, in which the melting material 61 , for example molten metal or a molten metal alloy, is located. About the sem melting 61 , a melting electrode 62 is arranged, which is attached to a support rod 63 which protrudes through an opening in the crucible and is locked there by means of a flange 64 . The protruding from the crucible 60 part of the support rod 63 is provided with a Ge thread 65 , which is guided by a drive nut 66 . This drive nut 66 is connected to a gear 67 , which in turn is coupled to a motor 68 which drives a tachometer generator 69 . A speed control device 70 acts on the motor 68 and is in turn acted upon by signals from the tachometer generator 69 . On the crucible 60 there is a vacuum pump system 71 which keeps the inside of the crucible 60 at a predetermined low pressure. Between the bottom of the crucible 60 and the Hal testange 63 of the electrode 62 , a power supply 72 is connected, which applies a voltage between the end of the electrode and the surface of the melting material 61 , the so-called arc voltage. The actual value of the arc voltage is passed to a direct current transformer 73 , the output signal U _{Li of} which is fed to a logic element 74 , which also receives the desired arc value U _LS , which is tapped by a potentiometer 75 .

The difference between the actual and the target value of the arc voltage is fed to a voltage regulator 75 , which gives a control signal to the speed control 70 via a switch 76 . The actual droplet sequence control is switched to the speed control 70 via another switch 77 . It contains a trigger 78 which is connected to the direct current transformer 73 and which triggers the drop short-circuit pulses coming from there. The pulses emitted by the trigger 78 can still differ in amplitude and / or pulse width and are therefore supplied to a standard pulse generator 79 which forms pulses of a uniform shape from them. The essential characteristic of the pulses coming from the standard pulse generator is therefore only their time interval, ie the pulse train. The mean value of all the pulses within a predetermined time interval is then formed in a downstream averager 80 . The decisive step of the present invention is that a reciprocal value generator 81 is provided which forms the reciprocal value from the output signal of the mean value generator 80 . If the averager 80 has determined a total of X drop short-circuit pulses within T seconds, the pulse rate for this period is X / T. In the reciprocal value generator 81 , this value is reversed in terms of amount, ie I / X per T is calculated. This reciprocal value is then fed as an actual value to a logic element 82 , to which a setpoint, which is tapped at a potentiometer 83 , is simultaneously fed. The difference between the actual value and the target value then reaches a drop follower 84 which is connected to the speed control 70 via the switch 77 already mentioned.

As a comparison of FIGS. 2 and 3 shows, the arrangement according to FIG. 2 results in a non-linear circular gain. Measurements carried out (cf. for example FIG. 5 of the mentioned patent application by Sandia Corp., or FIG. 5a) show that the mean drop frequency, ie the mean time interval between two drops, is approximately a linear function of the arc length. Since the characteristic value of the drop rate is the drop sequence, the drop rate thus has a hyperbolic connection with the arc length (see FIG. 5b). If a controller is now used which uses the drop frequency (rate) as the controlled variable, as shown in FIG. 2, a control loop with a non-constant loop gain is obtained when the system is included. In order to avoid oscillation, the size of the control intervention must be severely restricted if there is a deviation from the setpoint. A small loop gain must therefore be selected. This means sluggish disturbance control with large deviations from the setpoint.

If, on the other hand, as in the arrangement according to FIG. 3, the reciprocal value is formed from the output signal of the mean value generator 80 , the deviation signal is proportional to the distance deviation. The time constant and the loop gain are constant, ie the controller can be set optimally.

FIG. 4 shows a digital version of the arrangement shown in FIG. 3, the upper region being omitted. It can be seen here that only a pre-memory 90 is connected between the standard pulse generator 79 and averager 80 and a digital-to-analog converter 11 to the output of the drip sequence controller 84 . The pre-memory 90 sums up all the pulses that accumulate during a cycle time of the digital evaluation device. It reads its memory content to the averager ( 80 ) at the end of the cycle time.

Claims (10)

1. Device for regulating the distance of a melting electrode to the surface of the melting material in a vacuum arc furnace, the short circuits resulting from drops between the melting electrode and the surface of the melting material being used as a control criterion, and the short circuits occurring within a predetermined period of time being determined and an averager can be supplied, which is connected to a controller which controls an electric drive for the fusible electrode, characterized in that the controller is a drop follower controller ( 84 ) and is acted upon by a signal which is the difference between the reciprocal of one of the averaging devices ( 80 ) formed drop rate signal and a drop sequence setpoint. 2. Device according to claim 1, characterized in that the mean value generator ( 80 ) is connected upstream of a standard pulse generator ( 79 ) which is controlled by a trigger ( 78 ) which detects the drop short signal. 3. Device according to claim 1, characterized in that the drop follower ( 84 ) via a switch ( 77 ) with a speed control device ( 70 ) can be connected, and that this speed control device ( 70 ) simultaneously via a further switch ( 76 ) a voltage regulator ( 75 ) for the arc voltage can be connected. 4. Device according to claim 3, characterized in that the speed control device ( 70 ) controls a motor ( 68 ) which drives a transmission ( 67 ) which is connected to a holder ( 63 ) of the electrode ( 62 ). 5. Device according to claim 2, characterized in that a monostable multivibrator ( 39 , 39 ') is provided as the Normim pulse generator ( 79 ). 6. Device according to claim 1, characterized in that a PT ₁ member is provided as a value-forming agent. 7. Device according to claim 1, characterized in that as a means a ring counter is provided which calculates the average over all of the ring times detected short circuits. 8. The device according to claim 1, characterized in that the mean value generator ( 80 ), the reciprocal value generator ( 81 ), the target value specification ( 83 ) and the drip-sequence controller ( 84 ) work digitally and the output variables subsequently in a digital-to-analog converter analog size for controlling the feed motor is converted. 9. Device according to claim 1 and 8, characterized in that the Sig nal processing hybrid, i.e. partly digital, partly analog. 10. Device according to claim 8 and 9, characterized in that a pre-memory ( 90 ) is set in front of the averager ( 80 ), which accumulates all impulses during a cycle time (z. B. 100 ms) of the digital evaluation device and its memory content reads out to the averager ( 80 ) at the end of the cycle time.